Differential post-transcriptional regulation of two Ink4 proteins, p18 Ink4c and p19 Ink4d

Cyclin(-D-)-dependent kinase (Cdk) inhibitors of the Ink4 family specifically bind to Cdk4 and Cdk6, but not to other Cdks. Ink4c and Ink4d mRNAs are maximally and periodically expressed during the G(2)/M phase of the cell division cycle, but the abundance of their encoded proteins is regulated through distinct mechanisms. Both proteins undergo polyubiquitination, but the half life of p18(Ink4c) (approximately 10 hours) is much longer than that of p19(Ink4d) (approximately 2.5 hours). Lysines 46 and 112 are preferred sites of ubiquitin conjugation in p18(Ink4c), although substitution of these and other lysine residues with arginine, particularly in combination, triggers protein misfolding and accelerates p18(Ink4c) degradation. When tethered to either catalytically active or inactive Cdk4 or Cdk6, polyubiquitination of p18(Ink4c) is inhibited, and the protein is further stabilized. Conversely, in competing with p18(Ink4c) for binding to Cdks, cyclin D1 accelerates p18(Ink4c) turnover. In direct contrast, polyubiquitination of p19(Ink4d) is induced by its association with Cdks, whereas cyclin D1 overexpression retards p19(Ink4d) degradation. Although it has been generally assumed that p18(Ink4c) and p19(Ink4d) are biochemically similar Cdk inhibitors, the major differences in their stability and turnover are likely key to understanding their distinct biological functions.

The INK4-ARF (CDKN2A/B) locus in hematopoiesis and BCR-ABL-induced leukemias

Senescence and apoptosis programs governed by the Rb and p53 signaling networks can counter tissue stem cell self-renewal. A master regulator of Rb and p53 is the INK4-ARF (CDKN2A/B) locus that encodes two CDK inhibitors, p16(INK4A) and p15(INK4B), that maintain Rb in its active, hypophosphorylated form, and p14(ARF) (p19(Arf) in mice), that inhibits Mdm2 and activates p53. The INK4-ARF genes are epigenetically silenced in hematopoietic stem cells but become poised to respond to oncogenic stress as blood cells differentiate. Inactivation of INK4-ARF endows differentiated cells with an inappropriate self-renewal capacity, a defining feature of cancer cells. In BCR-ABL-induced (Philadelphia chromosome-positive [Ph(+)]) leukemias, INK4-ARF deletions frequently occur in clinically aggressive acute lymphoblastic leukemias (Ph(+) ALLs) but are not seen in more indolent Ph(+) chronic myelogenous leukemia (CML) or in CML myeloid blast crisis. Mouse modeling of Ph(+) ALL reveals that Arf inactivation attenuates responsiveness to targeted BCR-ABL kinase inhibitors, enhances the maintenance of leukemia-initiating cells within the hematopoietic microenvironment, and facilitates the emergence of malignant clones that harbor drug-resistant BCR-ABL kinase mutations. Thus, although BCR-ABL mutations typify drug resistance in both CML and Ph(+) ALL, loss of INK4-ARF in Ph(+) ALL enhances disease aggressiveness and undermines the salutary effects of targeted therapy.

Arf-induced turnover of the nucleolar nucleophosmin-associated SUMO-2/3 protease Senp3

The stabilization and subcellular localization of the p19(Arf) tumor suppressor protein and the SUMO-2/3 deconjugating protease Senp3 each depend upon their binding to the abundant nucleolar protein nucleophosmin (Npm/B23). Senp3 and p19(Arf) antagonize each other's functions in regulating the SUMOylation of target proteins including Npm itself. The p19(Arf) protein triggers the sequential phosphorylation, polyubiquitination and rapid proteasomal degradation of Senp3, and this ability of p19(Arf) to accelerate Senp3 turnover also depends on the presence of Npm. In turn, endogenous p19(Arf) and Senp3 are both destabilized in viable Npm-null mouse embryo fibroblasts (that also lack p53), and reintroduction of the human NPM protein into these cells reverses this phenotype. NPM mutants that retain their acidic and oligomerization domains can re-stabilize both p19(Arf) and Senp3 in this setting, but the nucleolar localization of NPM is not strictly required for these effects. Knockdown of Senp3 with shRNAs mimics the antiproliferative functions of p19(Arf) in cells that lack p53 alone or in triple knock-out cells that lack the Arf, Mdm2 and p53 genes. These findings reinforce the hypothesis that the p53-independent tumor suppressive functions of p19(Arf) may be mediated by its ability to antagonize Senp3, thereby inducing cell cycle arrest by abnormally elevating the cellular levels of SUMOylated proteins.

Failure of CDKN2A/B (INK4A/B-ARF)-mediated tumor suppression and resistance to targeted therapy in acute lymphoblastic leukemia induced by BCR-ABL

Deletions of the CDKN2A/B tumor suppressor locus and of the IKAROS and PAX5 genes that promote B-lineage development occur frequently in lymphoid, but not myeloid leukemias initiated by the BCR-ABL tyrosine kinase. Why is this the case, and how do these genetic lesions contribute to an aggressive disease that fails to durably respond to targeted kinase inhibitors?

Regulation of CYL/cyclin D genes by colony-stimulating factor 1

The proliferative effects of colony-stimulating factor 1 (CSF-1) on macrophages are exerted only throughout the G1 phase of the cell cycle. Genetic targets of the delayed early response to CSF-1 include novel G1 cyclin (CYL or cyclin D) genes. In macrophages, cyclin D1 is induced early in G1 and is expressed throughout the cell cycle as long as CSF-1 is present. The cyclin D1 protein turns over rapidly in CSF-1-stimulated cells and its level declines precipitously upon CSF-1 withdrawal. Cyclin D2 is induced later in G1 and its expression is periodic, whereas cyclin D3 is not expressed in macrophages but is regulated by growth factors in other cell types. The cyclin D1 protein associates during G1 with a polypeptide antigenically related to p34cdc2 and binds in vitro to a histone H1 kinase present in lysates of CSF-1-starved macrophages. The instability of the cyclin D1 protein and its ability to rescue a cyclin-dependent kinase activity from growth factor-deprived macrophages together suggest that the cyclin D protein is the dynamic partner in the complex. The timing of expression of cyclin D genes suggests that they act to link growth factor signals with cell cycle transitions during G1.

Signal-response coupling mediated by the transduced colony-stimulating factor-1 receptor and its oncogenic fms variants in naive cells

Colony-stimulating factor-1 (CSF-1 or M-CSF) supports the proliferation and survival of mononuclear phagocytes by binding to a receptor (CSF-1R) encoded by the c-fms proto-oncogene. Whereas the CSF-1R kinase is normally regulated by ligand, receptors bearing 'activating mutations' act constitutively as enzymes and can transform fibroblasts and haemopoietic cells of different lineages. Introduction of human CSF-1R enables mouse NIH-3T3 cells to form colonies in agar in response to human CSF-1 and to proliferate in serum-free medium supplemented with CSF-1, albumin, transferrin and insulin. Similarly, expression of human CSF-1R in interleukin 3-dependent mouse FDC-P1 myeloid cells enables them to grow in CSF-1. High levels of CSF-1R expression in FDC-P1 cells can induce factor-independent growth which is abrogated by a 'neutralizing' monoclonal antibody to the receptor. Therefore, critical mutations in the c-fms gene or overexpression of CSF-1R in immature myeloid precursors might each contribute to leukaemia.

The ARF tumor suppressor in acute leukemias: insights from mouse models of Bcr-Abl-induced acute lymphoblastic leukemia

The prototypical Bcr-Abl chimeric oncoprotein is central to the pathogenesis of chronic myelogenous leukemias (CMLs) and a subset of acute lymphoblastic leukemias (Ph+ ALLs). The constitutive tyrosine kinase transforms either hematopoietic stem cells (in CML) or committed pre-B lymphoid progenitors (in Ph+ ALL) to generate these distinct diseases. The INK4A/ARF tumor suppressor locus is frequently deleted in both B- and T-lineage ALLs, including Ph+ ALL, whereas the locus remains intact in CML. In murine bone marrow transplant models and after transfer of syngeneic Bcr-Abl-transformed pre-B cells into immunocompetent recipient animals, Arf gene inactivation dramatically decreases the latency and enhances the aggressiveness of Bcr-Abl-induced lymphoblastic leukemia. Targeted inhibition of the Bcr-Abl kinase with imatinib provides highly effective therapy for CML, but Ph+ ALL patients do not experience durable remissions. Despite exquisite in vitro sensitivity of Arf-null, BCR-ABL+ pre-B cells to imatinib, these cells efficiently establish lethal leukemias when introduced into immunocompetent mice that receive continuous, maximal imatinib therapy. Bcr-Abl confers interleukin-7 (IL-7) independence to pre-B cells, but imatinib treatment restores the requirement for this cytokine. Hence, IL-7 can reduce the sensitivity of Bcr-Abl+ pre-B cells to imatinib. Selective inhibitors of both Bcr-Abl and the IL-7 transducing JAK kinases may therefore prove beneficial in treating Ph+ ALL.

Cytokine-dependent imatinib resistance in mouse BCR-ABL+, Arf-null lymphoblastic leukemia

Retroviral transduction of the BCR-ABL kinase into primary mouse bone marrow cells lacking the Arf tumor suppressor rapidly generates polyclonal populations of continuously self-renewing pre-B cells, virtually all of which have leukemic potential. Intravenous infusion of 20 such cells into healthy syngeneic mice induces rapidly fatal, transplantable lymphoblastic leukemias that resist imatinib therapy. Introduction of BCR-ABL into Arf-null severe combined immunodeficient (SCID) bone marrow progenitors lacking the cytokine receptor common gamma-chain yields leukemogenic pre-B cells that exhibit greater sensitivity to imatinib in vivo. Hence, salutary cytokines in the hematopoietic microenvironment can facilitate leukemic proliferation and confer resistance to targeted therapy.

Genetic Alterations in Mouse Medulloblastomas and Generation of TumorsDe novofrom Primary Cerebellar Granule Neuron Precursors

Mice lacking p53 and one or two alleles of the cyclin D-dependent kinase inhibitor p18(Ink4c) are prone to medulloblastoma development. The tumor frequency is increased by exposing postnatal animals to ionizing radiation at a time when their cerebella are developing. In irradiated mice engineered to express a floxed p53 allele and a Nestin-Cre transgene, tumor development can be restricted to the brain. Analysis of these animals indicated that inactivation of one or both Ink4c alleles did not affect the time of medulloblastoma onset but increased tumor invasiveness. All such tumors exhibited complete loss of function of the Patched 1 (Ptc1) gene encoding the receptor for sonic hedgehog, and many exhibited other recurrent genetic alterations, including trisomy of chromosome 6, amplification of N-Myc, modest increases in copy number of the Ccnd1 gene encoding cyclin D1, and other complex chromosomal rearrangements. In contrast, medulloblastomas arising in Ptc1(+/-) mice lacking one or both Ink4c alleles retained p53 function and exhibited only limited genomic instability. Nonetheless, complete inactivation of the wild-type Ptc1 allele was a universal event, and trisomy of chromosome 6 was again frequent. The enforced expression of N-Myc or cyclin D1 in primary cerebellar granule neuron precursors isolated from Ink4c(-/-), p53(-/-) mice enabled the cells to initiate medulloblastomas when injected back into the brains of immunocompromised recipient animals. These "engineered" tumors exhibited gene expression profiles indistinguishable from those of medulloblastomas that arose spontaneously. These results underscore the functional interplay between a network of specific genes that recurrently contribute to medulloblastoma formation.

Regulation of the Arf tumor suppressor in Emicro-Myc transgenic mice: longitudinal study of Myc-induced lymphomagenesis

Lymphomagenesis in Emu-Myc mice is opposed by the Arf tumor suppressor, whose inactivation compromises p53 function and accelerates disease. Finding nascent Emu-Myc-induced tumors in which p19Arf causes cell-cycle arrest or apoptosis is problematic, since such cells will be eliminated until Arf or p53 function is lost. Knock-in mice expressing a green fluorescent protein (GFP) in lieu of Arf coding sequences allow analysis of Arfpromoter regulation uncoupled from p19Arf action. Prior to frank lymphoma development, unexpectedly low levels of Emu-Myc-induced p19Arf or GFP were expressed. However, as lymphomas arose in Arf+/GFP heterozygotes, additional oncogenic events synergized with Emu-Myc to further induce the functionally null Arf-Gfp allele. Concomitant up-regulation of p19Arf was not observed; instead, the wild-type allele was inactivated. We infer that very low levels of Arf are tumor suppressive, and that further induction provides the selective pressure for the emergence of tumors that have inactivated the gene.

p53-Dependent and -independent functions of the Arf tumor suppressor

The Ink4a-Arf locus encodes two closely wedded tumor suppressor proteins (p16(Ink4a) and p19(Arf)) that inhibit cell proliferation by activating Rb and p53, respectively. With few exceptions, the Arf gene is repressed during mouse embryonic development, thereby helping to limit p53 expression during organogenesis. However, in adult mice, sustained hyperproliferative signals conveyed by somatically activated oncogenes can induce Arf gene expression and trigger a p53 response that eliminates incipient cancer cells. Disruption of this tumor surveillance pathway predisposes to cancer, and inactivation of INK4a- ARF by deletion, silencing, or mutation has been frequently observed in many forms of human cancer. Although it is accepted that much of Arf's tumor-suppressive activity is mediated by p53, more recent genetic evidence has pointed to additional p53- independent functions of Arf, including its ability to inhibit gene expression by a number of other transcription factors. Surprisingly, the enforced expression of Arf in mammalian cells promotes the sumoylation of several Arf-interacting proteins, implying that Arf has an associated catalytic activity. We speculate that transcriptional down-regulation in response to Arf-induced sumoylation may account for Arf's p53-independent functions.

Divorcing ARF and p53: an unsettled case

Mammalian cells that sustain oncogenic insults can invoke defensive programmes that either halt their division or trigger their apoptosis, but these countermeasures must be finely tuned to discriminate between physiological and potentially harmful growth-promoting states. By functioning specifically to oppose abnormally prolonged and sustained proliferative signals produced by activated oncogenes, the ARF tumour suppressor antagonizes functions of MDM2 to induce protective responses that depend on the p53 transcription factor and its many target genes. However, ARF has been reported to physically associate with proteins other than MDM2 and to have p53-independent activities, most of which remain controversial and poorly understood.

N-Myc and the cyclin-dependent kinase inhibitors p18Ink4c and p27Kip1 coordinately regulate cerebellar development

Conditional N-Myc deletion limits the proliferation of granule neuron progenitors (GNPs), perturbs foliation, and leads to reduced cerebellar mass. We show that c-Myc mRNA levels increase in N-Myc-null GNPs and that simultaneous deletion of both c- and N-Myc exacerbates defective cerebellar development. Moreover, N-Myc loss has been shown to trigger the precocious expression of two cyclin-dependent kinase inhibitors, Kip1 and Ink4c, in the cerebellar primordium. We now further demonstrate that the engineered disruption of the Kip1 and Ink4c genes in N-Myc-null cerebella partially rescues GNP cell proliferation and cerebellar foliation. These results provide definitive genetic evidence that expression of N-Myc and concomitant down-regulation of Ink4c and Kip1 contribute to the proper development of the cerebellum.

Tbx3, the ulnar-mammary syndrome gene, and Tbx2 interact in mammary gland development through a p19Arf/p53-independent pathway

The closely related T-box genes Tbx2 and Tbx3 are both expressed in the developing mammary glands of mouse embryos and both have been implicated in mammary carcinogenesis. Tbx3 is essential for induction of the mammary placodes in mice. In humans, mutations in TBX3 are responsible for ulnar-mammary syndrome. Here, we show a haploinsufficiency effect of Tbx3 on maintenance of the mammary placodes and on the extent of branching of the ductal tree in mice. Loss or heterozygosity for Tbx2, on the other hand, has no effect on either induction or maintenance of the placodes, although a small effect was seen on branching morphogenesis in adult heterozygotes. However, the deficiency in maintenance of the mammary placodes in Tbx2, Tbx3 double heterozygous mice is more marked than in Tbx3 single heterozygotes, indicating a genetic interaction between the two genes. In spite of a large body of evidence implicating these genes in cell cycle control through the p19(Arf)/p53 pathways, we find no evidence for involvement of these pathways either in embryonic lethality of homozygous mutants or in the mammary gland phenotype of Tbx3 heterozygous mice.

Autophagy by ARF: a short story

In this issue of Molecular Cell, Reef et al. (2006) describe a shortened unstable form of the ARF tumor suppressor protein that localizes within mitochondria, where it reduces membrane potential and triggers autophagy. Could this account for the Mdm2- and p53-independent tumor suppressive effects of ARF?

Arf gene loss enhances oncogenicity and limits imatinib response in mouse models of Bcr-Abl-induced acute lymphoblastic leukemia

Mouse bone marrow cells transduced with retroviral vectors encoding either of two oncogenic Bcr-Abl isoforms (p210(Bcr-Abl) and p185(Bcr-Abl)) induce B cell lympholeukemias when transplanted into lethally irradiated mice. If the activity of the Arf tumor suppressor is compromised, these donor cells initiate a much more highly aggressive and rapidly fatal disease. When mouse bone marrow cells expressing Bcr-Abl are placed in short-term cultures selectively designed to support the outgrowth of pre-B cells, only those lacking one or two Arf alleles can initiate lympholeukemias when inoculated into immunocompetent, syngeneic recipient mice. Although the ABL kinase inhibitor imatinib mesylate (Gleevec) provides highly effective treatment for BCR-ABL-positive chronic myelogenous leukemia, it has proven far less efficacious in the treatment of BCR-ABL-positive acute lymphoblastic leukemias (ALLs), many of which sustain deletions of the INK4A-ARF (CDKN2A) tumor suppressor locus. Mice receiving Arf-/- or Arf+/- p210(Bcr-Abl)-positive pre-B cells do not achieve remission when maintained on high doses of oral imatinib therapy and rapidly succumb to lympholeukemia. Although cells expressing the Bcr-Abl kinase can proliferate in the absence of IL-7, they remain responsive to this cytokine, which can reduce their sensitivity to imatinib. Treatment of Arf-/-, p210(Bcr-Abl)-positive pre-B cells with imatinib together with an inhibitor of JAK kinases abrogates this resistance, suggesting that this combination may prove beneficial in the treatment of BCR-ABL-positive acute lymphoblastic leukemia.

Ubiquitination of, and sumoylation by, the Arf tumor suppressor

The Ink4a-Arf locus, which encodes two distinct tumor suppressor proteins, is inactivated in many cancers. Whereas p16Ink4a is an inhibitor of cyclin D-dependent kinases, p19Arf (p14ARF in humans) antagonizes the E3 ubiquitin protein ligase activity of Mdm2 to activate p53. We now recognize that Arf functions in both p53-dependent and -independent modes to counteract hyper-proliferative signals originating from proto-oncogene activation, but its p53-independent activities remain poorly understood. Arf proteins are highly basic (> 20% arginine content, pl > 12) and predominantly localize within nucleoli in physical association with an abundant acidic protein, nucleophosmin (NPM/B23). When bound to NPM, Arf proteins are relatively stable with half-lives of 6-8 hours. Although mouse p19Arf contains only a single lysine residue and human p14ARF has none, both proteins are N-terminally ubiquitinated and degraded in proteasomes. Through as yet uncharacterized mechanisms, p19Arf induces p53-independent sumoylation of a variety of cellular target proteins with which it interacts, including both Mdm2 and NPM. A naturally occurring NPM mutant (NPMc) expressed in myeloid leukemia cells redirects both wild-type NPM and p19Arf to the cytoplasm, inhibits Arf-induced sumoylation, and attenuates p53 activity. Thus, ubiquitination and sumoylation can each influence Arf tumor suppressor activity.

Hzf, a p53-responsive gene, regulates maintenance of the G2 phase checkpoint induced by DNA damage

The hematopoietic zinc finger protein, Hzf, is induced in response to genotoxic and oncogenic stress. The Hzf protein is encoded by a p53-responsive gene, and its overexpression, either in cells retaining or lacking functional 53, halts their proliferation. Enforced expression of Hzf led to the appearance of tetraploid cells with supernumerary centrosomes and, ultimately, to cell death. Eliminating Hzf mRNA expression by use of short hairpin (sh) RNAs had no overt effect on unstressed cells but inhibited the maintenance of G2 phase arrest following ionizing radiation (IR), thereby sensitizing cells to DNA damage. Canonical p53-responsive gene products such as p21Cip1 and Mdm2 were induced by IR in cells treated with Hzf shRNA. However, the reduction in the level of Hzf protein was accompanied by increased polyubiquitination and turnover of p21Cip1, an inhibitor of cyclin-dependent kinases whose expression contributes to maintaining the duration of the G2 checkpoint in cells that have sustained DNA damage. Thus, two p53-inducible gene products, Hzf and p21Cip1, act concomitantly to enforce the G(2) checkpoint.

The CDK Inhibitor p18Ink4c is a Tumor Suppressor in Medulloblastoma

Medulloblastoma (MB) is the most common malignant pediatric brain tumor which is thought to originate from cerebellar granule cell precursors (CGNPs) that fail to properly exit the cell cycle and differentiate. Although mutations in the Sonic Hedgehog (Shh) signaling pathway occur in 30% of cases, genetic alterations that account for MB formation in most patients have not yet been identified. We recently determined that the cyclin D-dependent kinase inhibitor, p18(Ink4c), is expressed as CGNPs exit the cell cycle, suggesting that this protein might play a central role in arresting the proliferation of these cells and in timing their subsequent migration and differentiation. In mice, disruption of Ink4c collaborates independently with loss of p53 or with inactivation of the gene (Ptc1) encoding the Shh receptor, Patched, to induce MB formation. Whereas loss of both Ink4c alleles is required for MB formation in a p53-null background, Ink4c is haplo-insufficient for tumor suppression in a Ptc(1+/-) background. Moreover, MBs derived from Ptc(1+/-) mice that lack one or two Ink4c alleles retain wild-type p53. Methylation of the INK4C (CDKN2C) promoter and complete loss of p18(INK4C) protein expression were detected in a significant fraction of human MBs again pointing toward a role for INK4C in suppression of MB formation.

The tumor suppressors Ink4c and p53 collaborate independently with Patched to suppress medulloblastoma formation

Recurrent genetic alterations in human medulloblastoma (MB) include mutations in the sonic hedgehog (SHH) signaling pathway and TP53 inactivation (approximately 25% and 10% of cases, respectively). However, mouse models of MB, regardless of their initiating lesions, generally depend upon p53 inactivation for rapid onset and high penetrance. The gene encoding the cyclin-dependent kinase inhibitor p18(Ink4c) is transiently expressed in mouse cerebellar granule neuronal precursor cells (GNPs) as they exit the cell division cycle and differentiate. Coinactivation of Ink4c and p53 provided cultured GNPs with an additive proliferative advantage, either in the presence or absence of Shh, and induced MB with low penetrance but with greatly increased incidence following postnatal irradiation. In contrast, mice lacking one or two functional Ink4c alleles and one copy of Patched (Ptc1) encoding the Shh receptor rapidly developed MBs that retained wild-type p53. In tumor cells purified from double heterozygotes, the wild-type Ptc1 allele, but not Ink4c, was inactivated. Therefore, when combined with Ptc1 mutation, Ink4c is haploinsufficient for tumor suppression. Methylation of INK4C (CDKN2C) was observed in four of 23 human MBs, and p18(INK4C) protein expression was extinguished in 14 of 73 cases. Hence, p18(INK4C) loss may contribute to MB formation in children.

Myeloid Leukemia-Associated Nucleophosmin Mutants Perturb p53-Dependent and Independent Activities of the Arf Tumor Suppressor Protein

Nucleophosmin (NPM or B23) plays key roles in ribosome biogenesis, centrosome duplication, and maintenance of genomic integrity. Mutations affecting the carboxylterminal domain of NPM occur in a significant percentage of adult patients with acute myeloid leukemia (AML), and these alterations create an additional nuclear export signal that relocalizes much of the protein from its normal nucleolar stores to the cytoplasm. When induced by oncogenic stress, the Arf tumor suppressor protein accumulates within the nucleolus, where it is physically associated with, and stabilized by, NPM. Ectopic overexpression of an NPM cytoplasmic mutant (NPMc) relocalized p19Arf and the endogenous NPM protein to the cytoplasm. NPMc-dependent export of p19Arf from the nucleus inhibited its functional interaction with the p53 negative regulator, Mdm2, and blunted Arf-induced activation of the p53 transcriptional program. Cytoplasmic NPM relocalization also attenuated Arf-induced sumoylation of Mdm2 and NPM and prevented wild type NPM from inhibiting p19Arf protein turnover. However, despite the ability of NPMc to interfere with these p53-dependent and independent activities of Arf, NPMc exhibited anti-proliferative activity in Arf-null NIH-3T3 cells. Overexpression of wild type NPM, but not NPMc, overcame premature senescence of Atm-null cells, a phenotype that can be rescued by inactivation of Arf or p53. Therefore, perturbation of Arf function appears to be insufficient to explain the oncogenic effects of the NPMc mutation. We favor the idea that NPMc also contributes to AML by dominantly perturbing other functions of the wild type NPM protein.

Sumoylation induced by the Arf tumor suppressor: a p53-independent function

The mouse p19(Arf) protein has both p53-dependent and p53-independent tumor-suppressive activities. Arf triggers sumoylation of many cellular proteins, including Mdm2 and nucleophosmin (NPM/B23), with which p19(Arf) physically interacts in vivo, and this occurs equally well in cells expressing or lacking functional p53. In an Arf-null NIH 3T3 cell derivative (MT-Arf cells) engineered to reexpress an Arf transgene driven by a zinc-inducible metallothionein promoter, sumoylation of endogenous Mdm2 and NPM proteins was initiated as p19(Arf) was induced and was observed before p53-dependent cell cycle arrest. Predominately nucleoplasmic molecules visualized by immunofluorescence with antibodies to small ubiquitin-like modifier (SUMO) 1 localized to nucleoli as p19(Arf) accumulated there. Two Arf mutants, one of which binds to Mdm2 and NPM but is excluded from nucleoli and the other of which enters nucleoli but is handicapped in binding to Mdm2 and NPM, were defective in inducing sumoylation of these two target proteins and did not localize bulk sumoylated molecules to nucleoli. The CELO adenovirus protein, Gam1, which inhibits the SUMO activating enzyme (E1) and leads to down-regulation of the SUMO conjugating enzyme (E2/Ubc9), had no overt effect on the ability of p19(Arf) to activate p53 or the p53-responsive genes encoding Mdm2 and p21(Cip1), despite the fact that Arf-induced sumoylation of Mdm2 was blocked. Reduction of Ubc9 levels with short hairpin RNAs rendered similar results. We suggest that Arf's p53-independent effects on gene expression and tumor suppression might depend on Arf-induced sumoylation.

Monoclonal antibodies to the mouse p19(Arf) tumor suppressor protein

The Arf tumor suppressor protein inhibits cell proliferation through both p53-dependent and -independent mechanisms. Two rat monoclonal antibodies raised against a peptide corresponding to amino acids 54-75 of the mouse p19(Arf) protein reacted with the native protein expressed in mammalian cells. Neither antibody detected human or golden hamster Arf proteins. The two antibodies, both of IgG2b isotype, are directed to adjacent epitopes contained within residues 54-62 and 62-75, respectively, of the mouse p19(Arf) protein, and both were highly efficient in detecting p19(Arf) by immunoprecipitation, immunoblotting, and immunofluorescence. One antibody proved useful for immunohistochemical staining of p19(Arf) in fixed sections of mouse testes, revealing low levels of protein expression within the nucleoli of spermatogonia. This indicates that these antibodies should be useful in detecting the endogenous p(19Arf) protein at specific stages of mouse development and during early stages of tumorigenesis.

Ras-Raf-Arf signaling critically depends on the Dmp1 transcription factor

Dmp1 prevents tumor formation by activating the Arf-p53 pathway. In cultured primary cells, the Dmp1 promoter was efficiently activated by oncogenic Ha-Ras(V12), but not by overexpressed c-Myc or E2F-1. Dmp1 promoter activation by Ras(V12) depended on Raf-MEK-ERK signaling. Induction of p19(Arf) and p21(Cip1) by oncogenic Raf was compromised in Dmp1-null cells, which were resistant to Raf-mediated premature senescence. A Ras(V12)-responsive element was mapped to the 5' leader sequence of the murine Dmp1 promoter, where endogenous Fos and Jun family proteins bind. Dmp1 promoter activation by Ras(V12) was strikingly impaired in c-Jun as well as in JunB knock-down cells, suggesting the critical role of Jun proteins in the activation of the Dmp1 promoter. A Ras(V12)-responsive element was mapped to the unique Dmp1/Ets site on the Arf promoter, where endogenous Dmp1 proteins bind upon oncogenic Raf activation. Therefore, activation of Arf by Ras/Raf signaling is indirectly mediated by Dmp1, explaining why Dmp1-null primary cells are highly susceptible to Ras-induced transformation. Our data indicate the presence of the novel Jun-Dmp1 pathway that directly links oncogenic Ras-Raf signaling and p19(Arf), independent of the classical cyclin D1/Cdk4-Rb-E2F pathway.